<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <style>
    #main {
      width: 0;
      height: 0;
      border: 30px solid;
      border-color: transparent transparent red transparent;
    }
  </style>
</head>
<body>
<div id="main"></div>
</body>
</html>
